This invention relates generally to a digital image scanner, and more particularly to a method and apparatus capable of achieving the reduction or elimination of image loss and distortion during the asynchronous operation of such a scanner.
The features of the present invention may be used in the printing arts and, more particularly, in a digital image acquisition system. It is commonly known to utilize an asynchronous communication link, or interface, between a digital image scanner and a host processor used to receive digital signals produced by such a scanner. While the asynchronous operation of the interface enables the host and scanner to operate at different image data transmission rates, such a mismatch requires the scanner to adapt to situations when the image signal output rate exceeds the image signal input rate of the host processor. This may occur as a result of other processing being carried on within the host, or may be for reasons as simple as the limited processing speed of a dedicated host or its interface.
While it is known that such a mismatch in input/output (I/O) data rates can exist, attempts to compensate for the delay of output on the scanner side of the interface have generally employed either complicated buffering schemes requiring large amounts of memory, control of the scanning speed in proportion to the data rate mismatch, or the ability to start and stop the scanning process. The first alternative would require that the scanner be able to store up to a complete page of scanned image signals in memory so that the host system would be able to operate at any speed to retrieve the data on a page-by-page basis. Unfortunately, due to the large amount of memory required and the cost of memory, this alternative is prohibitively expensive for many digital scanning system, even to store a portion of the digitized image signals.
The second alternative, that of regulating the speed of the scanner with respect to the speed of the host, or more appropriately, the backlog of digital data which has not been transferred to the host, is utilized in the Xerox.RTM.7650 Pro Imager.RTM. scanner and described by Bell in U.S. Pat. No. 4,748,514 (Issued May 31, 1988), the relevant portions of which are hereby incorporated by reference. Bell discloses a scanner with a movable scanning array, wherein the speed of the array, and thus the rate at which image is captured, is regulated as a function of the residual capacity of a small data buffer used to store the image signals prior to transmitting them across an interface. In this manner, the asynchronous scanner attempts to substantially match the image signal output rate with the image signal input rate of the workstation. While viable, this alternative requires a scanning array which is driven by a precision drive and an encoder which is used to constantly monitor the displacement of the scanning array. In general, this results in an expensive motor and drive control system that may have a maximum scanning speed limitation as a result of the precision required for the scanning array.
Hence, the last alternative, that of a scanning system capable of start/stop operation, would appear to be the most inexpensive of the alternatives. Unfortunately, the latter alternative also lends itself to the creation of errors in the capture of image signals whenever the array is started or stopped, unless an expensive encoding system is employed to monitor the relative movement between the scanning array and the original document being scanned.
Other approaches have been devised to compensate for the loss of image signals during an image scanning operation, of which the following disclosures may be relevant: